1. Field of the Invention
This invention relates to a power source device used with advantage for a camera control unit designed to supply power to a camera device. More particularly, it relates to a power source device designed to prevent destruction or self-destruction of electronic equipment on mistaken wiring or connection of a cable designed for supplying the power to the electronic equipment.
2. Description of the Related Art
There has been known an imaging system having a camera device for generating image signals of an object and a camera control unit (CCU) for processing the image signals with white balance processing or gamma correction. In this imaging system, the CCU performing the above signal processing also supplies power from an internal power source device to the camera device for reducing its weight and relieving the user of the load during imaging.
Referring to FIG. 1, a CCU 101 having the power source device furnishes power over a coaxial cable 102 to the camera device 103. The CCU 101 processes image signals supplied from the camera device 103 over the coaxial cable 102 and encodes the video signal to send the encoded image signals to a monitor device, not shown.
For preventing possible damage to incorrectly wired electronic equipments on power up after mistaken wiring or connection of the coaxial cable 102 thereto, the CCU 101 detects the current flowing in the camera device or incorrectly connected electronic equipments in order to terminate current supply if the current is abnormal current.
Therefore, possible presence of the mistaken wiring of the coaxial cable 102 cannot be detected unless the power source device in the camera device is turned on.
Moreover, on power up, the current flowing in the camera device 103 is transiently increased due to inrush current, as shown in FIG. 2. Thus, for detecting possible presence of the mistaken wiring, it is necessary to detect the current value in the steady-state of the current after lapse of approximately one second since the time of power up. However, if the current value is measured after the current s in its steady state, the risk is high that the resistor of the electronic equipment be burned out before the current assumes a steady state, or the CCU 101 be self-destroyed.
On the other hand, if the coaxial cable 102 is connected by mistake to an input terminal of a monitor device 104 terminated with 75 ohms such that the current to be supplied to the camera device 103 is supplied by the CCU 101 to the monitor device 104, a dc voltage of 25 V is supplied to the 75-ohm terminal resistor. Thus, the current of 25V/75 ohm=333 mA flows through this 75 ohm resistor.
The CCU 101 is designed so that, on detection of the 333 mA current, current supply to the monitor device 104 is halted if the threshold current of halting the power source is 200 mA. However, if approximately one second is allowed to lapse since power up until stabilization of the current value, a voltage of 25 V is impressed across the terminal resistor of the monitor device 104. At this time, the power consumption in the terminal resistor becomes 89.3 W thus burning out the terminal resistor without fail.
The CCU 101 is provided with a power source limiter or a fuse for preventing destruction of the power source itself. However, if the coaxial cable 102 between the camera device 103 and the CCU 101 is shorted, the power loss in the CCU 101 is significant, even if a power source limiter is use, such that thermal destruction of the power source cannot be prevented easily. For example, if the limiter becomes effective for the current of 300 mV for the dc voltage of 25V, the power loss of at least 7.5 W or higher is incurred.